This invention relates to use of instrument landing systems by instrument-rated aircraft and, more particularly, to verification of the integrity of in-space signals transmitted by an instrument landing system.
Reduced visibility, due to adverse weather conditions, may make it difficult or impossible for a pilot to rely upon visual observation of the landing site to safely land an aircraft. The problem has been addressed by Instrument Landing System (ILS) installations at many airports. Such systems rely upon transmission of a pattern of signals in space. Equipment installed on board instrument-rated aircraft is arranged to receive the ILS signals. On a simplified overview basis, reception by an aircraft of a particular ILS signal (i.e., representative of a specific feature of a pattern in space of ILS approach signals) is intended to indicate to the pilot that the aircraft is at that time at a particular point in space relative to an aircraft runway. Thus, if a pilot who cannot see the runway, nevertheless is informed of the current position of the aircraft relative to the runway, suitable action may be taken to initiate an approach to a safe landing.
However, if the ILS signal which the pilot would rely upon is, in fact, not available at its intended position in space, any error in that position may cause the pilot to be misinformed as to the position in space of the aircraft relative to the runway. Of course, if it is known in advance that signals receivable from an ILS are not currently reliable, a pilot may be advised not to rely upon such signals. ILS signals may be monitored periodically to ensure that signals are being accurately transmitted. However, transmission anomalies affecting signals post transmission are generally not discernable from the ground. Even if the signals available from a particular ILS at an airport could be accurately verified on a day to day basis, a pilot may have to assume there has been no degradation of signal integrity in the intervening period between verifications. In fact, changing atmospheric or ground conditions, as well as other conditions such as temporary standing of a large aircraft in the field of an ILS antenna, may affect ILS signal integrity at any time and for any duration of time. As a result, in the absence of truly current information a pilot may be faced with a choice of landing with reliance on ILS signals which he is not aware are providing inaccurate indications, or redirecting the landing to a different airport.
The subject of ILS signal integrity and the monitoring thereof has been previously addressed by the present inventor and by others. See, for example, the following and references cited therein. F. Marcum, Evaluation of Image-Type Glide Slope Performance in the Presence of Snow Cover, IEEE Transactions on Aerospace and Electronic Systems, Vol. 34, No. 1, 71-83, January 1998; F. Marcum, Design of an Image Radiation Monitor for ILS Glide Slope, IEEE Transactions on Aerospace and Electronic Systems, Vol. 34, No. 3, 836-843, July 1998.
Objects of the present invention are to provide new and improved systems and methods to enable in-flight verification of instrument landing system transmissions and such systems and methods which may have one or more of the following characteristics or capabilities:                current verification of accurate positioning in space of ILS signals representing approach points;        current indication of in-space positioning anomalies of ILS glide slope path signals;        pilot alerts on out of specification ILS signal disparities not discernable from ground locations;        on-board current verification of ILS signal reliability for landings;        use of available positional and other data for independent verification of ILS information;        cost effective on-board verification of ILS information on a current in-flight basis; and        automated operation with pilot alerts.        